1. Field of the Invention
This invention relates in general to subsea well assemblies, and in particular to a method and apparatus for installing a first string of casing in the well so as to utilize pressure control during the drilling for the first string of casing.
2. Description of the Prior Art
In subsea well drilling of the type concerned herein, the operator will first drill a large diameter hole to a selected depth. The operator then secures a string of conductor pipe to an outer wellhead housing and lowers the assembly into the well. The operator will pump cement down the conductor pipe, which flows back up the annulus to cement the conductor pipe in the well.
Then, the operator will drill the well to a second depth. Often this is handled by drilling a pilot hole. A riser will extend from the drilling vessel into the outer wellhead housing during drilling of the pilot hole for drilling fluid circulation. A diverter at the drilling vessel will divert drilling fluid returns in the event of high pressure being encountered.
Then the operator will remove the riser and run a larger diameter drill bit through the outer wellhead housing and conductor pipe to ream out the pilot hole. Normally, cuttings and drilling fluid will flow out open cement ports provided in the outer wellhead housing, and will not be returned back to the drilling vessel. Also, normally, there will be no means for controlling any high pressure that might occur during drilling of this second portion of the well.
After the second depth has been drilled and reamed, the operator will secure a string of casing to an inner wellhead housing and lower the assembly into the well. The inner wellhead housing seats within the outer wellhead housing. While running the inner wellhead housing and casing, no pressure control equipment is available as the riser will not be connected. The outer wellhead housing has a radially extending return flow passage extending through its sidewall. The operator pumps cement down the string of casing with the cement flowing back up the annulus between the casing and the conductor pipe. Cement returns flow up and out the cement return ports of the outer wellhead housing.
In some offshore drilling, a subsea template will be used for drilling a number of wells. The drilling rig may be located on a vessel which remains at the site for production, such as a tension leg platform. In such a case, the operator will not want to deposit a large amount of cuttings on the subsea template. In the prior art, a substantial amount of cuttings would flow out, the outer wellhead housing cement return ports during reaming of the second drilling phase of the well. Also, because of a remote possibility of a blowout, the operator may wish to have pressure control during all phases of drilling, including reaming of the pilot hole and running the first string of casing.